1. Field of the Invention
The present invention relates to a light emitting material in which generated electroluminescence is obtained by imparting an electric field, and to a light emitting device using the light emitting material. In particular, the present invention relates to a light emitting device using an organic compound in the light emitting material.
2. Description of the Related Art
Display devices using liquid crystals have a typical construction which uses a back light or a front light, and they display an image using that light. Liquid crystal display devices are employed as image display means in various types of electronic devices, but they have a structural disadvantage in that their angle of view is narrow. In contrast, display devices using light emitting materials in which electroluminescence is obtained have a wide angle of view, and are the focus of next generation display devices due to their superior visibility.
Light emitting elements using organic compounds as light emitting materials (hereafter referred to as organic light emitting elements) are structured by appropriately combining layers such as hole injecting layers, hole transporting layers, light emitting layers, electron transporting layers, and electron injecting layers, formed by organic compounds, between a cathode and an anode. Although hole injecting layers and hole transporting layers are represented here as being distinct, they are the same in that hole transportability (hole mobility) is their particularly important property. In order to make distinction convenient, the hole injecting layer is taken as the layer on the side contacting the anode, and the hole transporting layer is taken as the layer on the side contacting the light emitting layer. Also, the electron injecting layer is taken as the layer containing the cathode, and the electron transporting layer is taken as the layer on the side containing the light emitting layer. There are times when the light emitting layer also serves as an electron transporting layer, and therefore it is also referred to as a light emitting electron transporting layer. Light emitting elements formed by combining these types of layers show rectification characteristics, and have structures similar to diodes.
It is thought that structures that emit light by electroluminescence do so by a phenomenon in which electrons injected from the cathode and holes injected from the anode recombine in the layer made from the light emitting material (light emitting layer), forming excitons, and light is irradiated when the excitons return to the ground state. Fluorescence and phosphorescence exist as types of electroluminescence, and these are understood as light emission from a singlet state (fluorescence) and light emission from a triplet state (phosphorescence) in the excited state. The brightness of the light emitted reaches from several thousands to several tens of thousands of cd/m2, and therefore it is considered possible to apply electroluminescence in theory to display devices and the like. However, various types of degradation phenomena also exist, and several problems to their practical application remain.
It is thought that there are five causes of degradation to light emitting materials made from organic compounds, and to organic light emitting elements: 1) chemical degradation of the organic compound (via the excited state); 2) melting of the organic compound due to heat generated during driving; 3) dielectric breakdown originating in macro faults; 4) degradation of the electrodes or electrode/organic layer interfaces; and 5) degradation due to instabilities in the amorphous structure of the organic compound.
The above first to third causes of degradation are due to driving of the organic light emitting element. The generation of heat is inevitable because electric current flowing within the element is converted into joule heat. If the melting point of the organic compound, or the glass transition temperature, is low, it is thought that melting will occur. Further, the existence of pinholes or cracks within the organic compound will concentrate the electric field in those locations and cause dielectric breakdown. Degradation proceeds even if the light emitting element is maintained at room temperature with the fourth and the fifth causes. The fourth cause is known as dark spots, and is due to cathode oxidation and reactions with moisture. The fifth cause is due to the fact that all organic compounds used in the organic light emitting element are amorphous materials. It is thought that crystallization occurs during long term storage, changes by long time, and the generation of heat, and that there are almost no materials with a stable amorphous structure can be maintained.
Dark spots can be well suppressed by using better sealing techniques, but in practice, degradation is caused by a composite of the aforementioned reasons, and the situation is difficult in order to be uniformly understood. A method in which organic light emitting elements formed on a substrate are sealed airtight by a sealing material, and a drying agent is formed on the inside space, is known as a typical sealing technique. However, the phenomenon in which the electric current flowing in the organic light emitting elements decreases, along with a decrease in the brightness of emitted light, if a constant voltage is continually applied to the light emitting element is thought to originate in the physical properties of the organic compound.
Both low molecular weight organic compounds and high molecular weight organic compounds are known as organic compounds for forming organic light emitting elements. Compounds such as the following are known as examples of low molecular weight organic compounds: á-NPD (4,4′-bis(N-(1-naphthyl)-N-phenylamino)-biphenyl), which is a copper phthalocyanine (CuPc) aromatic amine-based material, and MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine) as hole injecting layers; and tris-8-quinolinolate aluminum complex(Alq3) as a light emitting layer. Among the high molecular weight organic light emitting materials, examples such as polyaniline and polythiophene derivatives (PEDOT) are known.
Looking at the diversity of materials, low molecular weight organic compounds manufactured by evaporation have a remarkable diversity compared to high molecular weight organic materials. However, whatever material is used, organic materials which can be made purely from only basic structural units are rare. Different kinds of bonds and impurities are mixed in by manufacturing processes, and there are also times when various additives such as pigments are added. Further, these types of materials include those which deteriorate due to moisture and which are easily oxidized. It is possible for moisture, oxygen, and the like to easily become mixed in from the atmosphere, and therefore caution is required in material handling.
It is known that when organic compounds are degraded by light, chemical bonds become double bonds and the structure changes to one containing oxygen (such as —OH, —OOH, >C═O, —COOH). It is therefore thought that the bonding state changes and degradation advances if the organic compounds are placed within an atmosphere containing oxygen, or if oxygen or H2O exist as impurities within the organic compounds.
Seen as a diode, one type of semiconductor element, it is known that impurities caused by oxygen form localized levels within the forbidden bands in semiconductor elements having semiconductor junctions, and that this causes junction leakage and drops in the carrier lifetime, thereby greatly reducing the semiconductor element properties.
FIG. 11 is a graph showing the distribution in the depth direction of oxygen (O), nitrogen (N), hydrogen (H), silicon (Si), and copper (Cu), measured by secondary ion mass spectroscopy (SIMS), in an organic light emitting element. The structure of the sample used in making the measurements is as follows: tris-8-quinolinolate aluminum complex (Alq3)/carbazole-based material (Ir(ppy)3+CBP)/copper phthalocyanine (CuPc)/oxide conductive material (ITO)/glass substrate. Alq3 contains oxygen within its molecular structure, as shown by the chemical formula (Chem 1) below.
Chem 1
On the other hand, Ir(ppy)3+CBP and CuPc are structured such that there is no oxygen contained within their molecules, as shown by the following chemical formulae (Chem 2 and Chem 3):
Chem 2
Chem 3
The oxygen concentration becomes high in the Alq3 region and in a certain part of the ITO region in the concentration distributions of each of the elements shown by FIG. 11 for this reason. Conversely, the oxygen concentration is reduced in the Ir(ppy)3+CBP and CuPc layers. However, ions are detected on the order of 7×102 counts/sec, and it can be confirmed that oxygen exists in regions at which oxygen is not expected to be.
The highest occupied molecular orbital (HOMO) level degenerates and therefore oxygen molecules are unique triplet state molecules at their base state. Normally, the process from triplet to singlet excitation becomes more difficult to occur due to forbidden transitions (spin prohibitions), and therefore singlet state oxygen molecules are not generated. However, if triplet excitation state molecules (3M*) having an energy state that is higher than the singlet state exist in the periphery of the oxygen molecules, then this can lead to a reaction in which singlet state oxygen molecules are generated, in accordance with energy transfers occurring as shown below.
Eq 1
Seventy-five percent of the molecular excitation state in the light emitting layers of organic light emitting elements is said to be triplet state. Therefore, if oxygen molecules are mixed within the organic light emitting elements, then the generation of singlet state oxygen molecules can be obtained by the energy transfer of Eq 1. Singlet excitation state oxygen molecules have ionic nature (bias to electric charge), and therefore it is thought that there is the possibility of reaction with the electric charge bias developing in the organic compound.
For example, methyl group in batho-cuproene (hereafter referred to as BCP) is an electron donor, and therefore carbon directly bonded to conjugate rings is positively electrified. As shown by Chem 4 below, singlet oxygen with ionic properties reacts with carbon which is positively electrified, and there is the possibility that carboxylic acid and hydrogen are formed as shown by Chem 5 below. As a result, it can be expected that the electron transportability will decrease.
Chem 4
Chem 5